Method and composition for chromium plating



United States Patent 2,992,171 METHOD AND COMPOSITION FOR CHROMIUM PLATlNG William D. MacLean, Oakville, Ontario, and Kenneth C. 7 Graham, Weston, Ontario, Canada, assignors to Gen- .eral Development Corporation, Miami, Fla., 2 corporation of Delaware No Drawing. Filed Apr. 4, 1960, Ser. No. 19,459

- 8 Claims. (Cl. 204-51) This is a continuation in part of application Serial No. 668,318 filed June 27, 1957, now abandoned.

This invention is directed to a new and novel plating solution with a composition that is adapted for the electrodeposition of a corrosion resistant chromium upon the surfaces of metal materials and articles.

More specifically, the invention is concerned with a chromium plating solution having a composition that has proven inherently advantageous and which was found to be instrumental in obtaining a highly corrosion resistant chromium deposit directly upon bare aluminum surfaces. It has been established that the chromium deposited from the solution herein disclosed produces a protective finish on aluminum that has withstood one hundred hours of standard salt spray tests without showing'signs of corrosion breakdown.

One of the objects of this invention is to provide a chromium electroplating bath of predetermined composition that will have the property and the facility to deposit an adherent chromium plate directly upon or over the surface of an aluminum article with the formed plate consisting of such a high grade fine grained deposit that it is capable of withstanding corrosion under salt spray tests for at least one hundred hours duration conducted under known standard specifications.

Another object of this invention is to provide a chromium electroplating bath for chromium deposit directly upon aluminum that is capable of providing a plate that will not scale, peel or spall under extreme bending and deformation of the aluminum base material.

A further object of the invention is to provide a plating bath for chromium deposition upon aluminum wherein the smoothness of the deposited plate is of such a quality that the as plated chromium surface can be bright or that it possesses such a fine grain structure that only a touch of a polishing Wheel or similar instrumentality will adequately color the article to provide a highly decorative finish on the aluminum.

Another object of the invention is to provide a dry chemical composition which is suitable for introduction in predetermined concentration into an aqueous solution containing sulfate ion, for example sulfuric acid, to provide an improved bath for plating chromium on a basis metal.

Further objects and advantages of the present inventive concept will hereinafter appear in this specification or become apparent from the following more detailed description of the plating solution, its composition and its use. 4

The solution herein developed is applicable for chromium plating aluminum to provide the highly corrosion resistant results by direct electrodeposition upon thesurface of the aluminum article. The aluminum article should be well cleaned and substantially free from oxide film to a minimum order when the article is subjected to immersion in the plating solution of the present composition for the most efficient and most advantageous final resusts.

A superior chromium plate is attained particularly when the power supply for the plating process is of the type described in copending application, Serial No. 818,329, filed June 5, 1959.

The invention is described with particular reference to the plating of aluminum because aluminum is a notoriously difiicult metal to plate. It should be understood, however, that the process and composition described herein may be used in the plating of chromium on other metal surfaces such as nickel, copper, steel and brass, zinc and zinc die casting alloys.

The process of the present invention may be used in plating an aluminum surface which has been first cleaned and treated in a conventional manner. However, we prefer to prepare the aluminum surface in accordance with the process described in our copending application, Serial No. 859,525, filed December 14, 1959.

Following a preliminary treatment of the aluminum surface, the article is plated by the process of this in vention, of which the following is an example:

Chromium plate in a -l70 F. bath comprising:

20 oz. CrO by wt./ gal. of water 0.15 oz. H 80 by wt./ gal. of Water 0.20 oz. amorphous SiO by Wt./ gal. of water The pretreated article is placed in the bath for /2 to 10 minutes. Then current in the form described below is applied at a density of 0.5 to 5 amp. per sq. in. until the thickness desired has been deposited. The chromic acid concentration can vary in the range of 15-45 oz. per gal.; the sulfate should be introduced to provide a CrO to $0., ratio of 80/1 to 200/1, the preferred range being /1 to /1; and the SiO concentration can vary from 0.0333 oz./gal. to 6.0 oz. per gal. with the preferred range being 0.13 to 0.3 oz. per gal. Plate at an average voltage sufficient to pass the current at the density desired. Practice shows that a voltage at the electrodes in the range of 3-9 volts will be necessary depending on many factors such as the spacing of the electrodes, surface area of the electrodes, etc.

An upper limit of 6.0 oz./gal. of SiO is set forth above. This upper limit is somewhat artificial, for actual practice of the invention indicates that the upper limit on the concentration of SiO is that which tends to render the chromium plating solution gelatinous. Even that point of undesirableconcentration will be variable depending upon other factors, the principal factor being the temperature at which the plating process is performed.

It should be understood, too, that the lower limit of 0.0333 oz./gal. is notso critical as to delineate the concentration below which no successful plate is attainable and above which every plate will be satisfactory. Other factors such as CrO concentration, temperature, current density and sulfate ratio will influence the level of minimum concentration of SiO below which plating on aluminum cannot satisfactorily be performed.

7 In this connection, it has been found that if the SiO concentration is maintained at at least approximately 0.20 oz./ga1., variables such as those set forth above become far less critical to the success of the chromium plating operation. Stated another way, if the concentrationv of SiO,, is not permitted to fall below 0.20 oz./gal., such factors as sulfate ratio and temperature can be permitted greater variation than is normal-1y desirable without detriment to the resultant chromium plate.

Further, there has been observed no criticality of relationship of SiO to any other factor such as CrO concentration, temperature, current density, etc. so long as the SiO concentration is maintained at at least approximately 0.2 oz./gal.

As indicated above, an objective of the invention has been to provide a dry composition which can be introduced into a bath containing sulfate ion, to provide the plating bath of the invention. The composition will be a combination of CrO and SiO As indicated by the concentration ranges of CrO and SiO;, in the example set forth above, the ratio of CrO to SiO in such a dry composition could be in the broad range of 2.5/1 to 1350/ 1. However, compositions at the extremities of this range would have extremely limited usefulness. Preferably therefore, the useful range of composition ratio should be approximately 80/1 to 200/1, the preferred ratio being approximately 100/1. A composition having a ratio of CrO to SiO of 100/1 will be useful whether the CrO concentration is 15 02/ gal. or 45 oz./ gal.

The silica to be used in the plating bath must be colloidal, that is, the silica must have an extremely fine particle size, so that when introduced into the plating solution it will be suspended in the solution rather than settling out of the solution. Such colloidal silica is commercially available under the name Cab-O-Sil and is a submicroscopic particulate silica prepared in a hot gaseous environment of about 1100 C. by the vapor phase hydrolysis of a silicon compound. This product is of high chemical purity, of extremely fine particle size and it is readily dispersible. The silica content on a moisture free basis is 99.0-99.7% with a negligible amount of Fe O to the order of 0.004%. The product is white with a particle size range of 0015-0020 micron and it has a specific gravity of 2.10. The pH aqueous dispersion) factor is 4.5-6.0.

The other commercial product is obtainable as silica and this is characterized as having an SiO content greater than 99% leaving little room for impurities of any kind with possibly some slight but negligible moisture content. This silica is rated as having a particle size of 10-20 rnillimicrons, a specific gravity of 2.2 to 2.3 and a pH factor of 5.3.

The following are two specific examples of a process of plating chromium on aluminum in accordance with the present invention.

Example 1 After the aluminm surface has been prepared as discussed above, immerse in a bath constituted by an aqueous solution of CrO H 50 and SiO- in the following amounts:

CrO oz./gal 27 CrO to H 50 ratio 130/1-150/1 SiO oz./gal 0.2

The article should be immersed in such bath which is maintained at a temperature of 123 -125 F. After a five minute dwell in the bath, a plating current of 1.0 to 1.5 a.s.i. is applied for twenty minutes. A nearly full bright chromium plate of approximately 0.1 mil thickness results. The plate requires only light color buffing that is, about 10 seconds of buffing wheel application to bring out the full bright, lustrous blue chrome appearance.

Example II Example II is identical to Example I except for the following conditions:

Temperature of bath F 135-140 Current density a.s.i. 1.5-2.0 SiO -oz./gal. 0.266 Minimum plate thickness mil 0.15

Example III CrO n1 gal 22.7-23.4 Gro /H ratio 1-125/1 Temperature F -2.5 Current density a.s.i 1.5 Time ..minutes 35-40 Immerse one minute before applying current. With no SiO additive present, the plates were unsatisfactory, being flaked as plated.

Example IV Sarne conditions as Example III except that 0.0333 oz./ gal. Si0 was added to the bath. One out of three samples was reasonably satisfactory, that is, was not flaked as plated. The remaining samples were flaked as plated.

Example V Same conditions as Example III except that 0.143 02/ gal. Si0 was added to the bath. An adherent, buffable chromium plate was deposited on all samples.

Examples VI and VII Same conditions as in Example III except that 0.2 oz./gal. and 0.667 oz./gal., respectively, of Si0 were added to the baths. Adherent, butfable chromium plates were deposited on all samples.

Example VIII The following conditions resulted in a buifa'ble, adherent corrosion resistant plate on an aluminum surface:

CrO oz./ gal- 20 H 80 oz./gal 0.2 Slog 07 /gal 0.2 Temperature F 15 2:2

Immerse for 30-60 seconds. Then apply a plating current of 2.08 a.s.i. for seven minutes and thereafter 1.39 a.s.i. for fifteen minutes to produce a minimum thickness of 0.1 mil of chromium.

Example IX In this example of the invention, chromium is plated over a bright nickel plate which has previously been deposited on a metal article by conventional methods.

(1) Bright nickel plate by conventional methods.

(2) Rinse.

(3) Acid dip in 6 N hydrochloric acid.

(4) Rinse.

(5) Plate for 5 minutes in a 110:5 F. aqueous solution constituted as follows:

CrO oz./gal 20-22 CrO /H SO ratio 130/1 SiO oz./gal .2 Current density a.s.i

The resulting plate is crack-free, non-porous and clear mirror bright.

Example X A high-temperature Fe-Cr-Al alloy (70% Fe, 25% Cr and 5% Al) was plated with about 1 mil of chromium using the following procedure:

(1) Descale (vapor blast or grind).

FeCl .4H O g./l 18s HCl (1.19 sp. gr.) ml./l 80 50 amp/sq. ft min 6 (9) Water rinse. Chromium plate 1.0 mil:

CrO 150 g./l. H SO 1.5 g./l. SiO 1.5 g./-l. 150 2.0 amp/sq. in. strike plate;

1.5 amp/sq. in. 3.5 hours. (11) Rinse and dry.

The chromium-plated alloy was heated in a Bunsen burner to about 1200" F. and quenched in water and also heated to 2100 F. for 3 hours with no evidence of spalling or flaking. The hardness of the chromium after heating was 230 Knoop hardness numbers. The chromium-plated alloy was bent back on itself and was cut with a band saw with no evidence of flaking.

Example XI 70-30 brass was cleaned by conventional means and plated in the bath of the present invention:

22 02/ gal. CrO

130/ l CrO /SO ratio .2 oz./ gal. SiO

Temp. 145 F.

1.5 amps. per sq. in. for 45 min-immerse with current on to plate about 0.30 mil.

The plate was non-porous, crack-free and buffable to a lustrous finish. CASS test for 18 hours had an ASTM rating of 10.

' Example XII Aluminum sheet of 3003 alloy was bufied on one side and treated by the following steps:

(1) Trichloroethylene degrease.

(2) Dip in a caustic cleaner, 2 oz./gal., 130 F., 30

seconds.

(3) Water rinse.

(4) Immerse in conditioning bath:

wt. percent H 80 0.1 g./l. chromium (added as Cr (SO xH O) 3.7 g./l. aluminum (added as A1 (SO .18H O) 2-minute immersion.

Water rinse.

Immerse in activating bath:

l30i2.5 F.

Plate in chromium plating bath:

Bath of step 8 minutes 1.5 amp/in.

(27 amperes per test panel) (10) Rinse and dry.

6 Example XIII Same steps as in Example XII except:

Step 8. Temperature ll0i2 F. Step 9. Temperature 1l0i2 F.

1.0 amp/in.

18 amps.

Example XIV Same steps as in Example XII except:

Steps 8 and 9 Additive concentration, 45 g./l.

Example XV Same steps as in Example XII except:

Steps 8 and 9 Additive concentration, 45 g./l.

It was noted that when the additive concentration was 45 g./l. (6 oz. per gal.) and the bath is at a temperature of 70 F., the bath is very viscous and the bottom of the bath inch jells. An aqueous composition containing at least 45 g./l. of SiO and properly proportioned CrO and H would be well suited for shipment from a chemical supplier to a plating plant. The added water, of course, would increase the weight and add to the shipping charge, although in relation to the value of the goods shipped the added freight charge would be negligible. The added freight charge would be offset by the fact that such form of shipment of SiO is much easier to handle than it is in its dry state, because in its dry particulate state it comprises such fine particles, it appears to be almost weightless and scatters about as dust when it is being introduced into a plating bath.

Having described our invention, we claim:

1. The method of electrodepositing a chromium plate directly on the surface of an aluminum article; which comprises immersing said aluminum article in a plating bath comprising a mixture of 15-45 oz./ gal. CrO H SO the ratio of CrO to H SO being in the range of 80/1 to 200/1, water, and 0.0333-6.0 oz./ gal. SiO as particles of submicroscopic dimensions carried in suspension in said bath; and electrolyzing said bath to plate said article.

2. The method of electrodepositing a chromium plate directly on the surface of an aluminum article; which comprises immersing said aluminum article in a plating bath comprising a mixture of 15-45 oz./ gal. CrO H 80 the ratio of C'rO to H SO being in the range of 80/ 1 to 200/ 1, water, and 0.03336.0 oz./gal. 'SiO as particles of colloidal dimensions carried in suspension in said bath; and electrolyzing said bath to plate said article.

3. A composition for use in an aqueous chromium plating solution, said composition comprising CrO and SiO the ratio of CrO to SiO being in the range of 25/1 to 1350/1, said SiO having a particle size of submicroscopic dimensions whereby said SiO is carried in suspension in said aqueous solution.

4. A composition for use in an aqueous chromium plating solution, said composition comprising CrO and SiO the ratio of 'CrO to SiO being in the range of 2.5/1 to 1350/1, said SiO having a particle size of colloidal dimensions whereby said SiO is carried in suspension in said aqueous solution.

5. A composition for use in an aqueous chromium plating solution, said composition comprising CrO and SiO the ratio of CrO to SiO being approximately 1, said SiO having a particle size of submicroscopic dimensions whereby said SiO is carried in suspension in said aqueous solution.

6. A composition for use in an aqueous chromium plating solution, said composition comprising CrO and SiO the ratio of CrO to SiO being approximately 100/ 1, said SiO having a particle size of colloidal dimensions whereby said 'Si0 is carried in suspension in said aqueous solution.

7. A chromium plating solution comprising, an aqueous solution of 15-45 oz./ gal. CrO H SO the ratio of Cr0 7 to H 50 being in the range of 80/1 to 200/1, and 0.033 3- 6.0 oZ./gal. SiO as particles of submicroscopic dimen sions carried in suspension in said solution,

8. A chromium plating solution comprising, an aqueous solution of 15-45 oz./ gal. CrO H 80 the ratio of CrO to H SO being in the range of 80/ 1 to 200/ 1, and 0.03336.0 oz./ gal. SiO as particles of colloidal dimensions carried in suspension in said solution.

References Cited in the file of this patent UNITED STATES PATENTS Dieffenbach Apr. 10, 1906 Auerbach Mar. 19, 1929 Proctor Mar. 4, 1930 MacLe'an May 27, 1958 Smith July 1, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 2,992,171 July 11, 1961 William D. MacLean et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 66, for "a.s.i;." read a.s.f.

Signed and sealed this 6th day of March 1962.

( SEA L) Attest:

ERNEST W. SWIDER v DAVID L. LADD Attesting Officer Commissioner of Patents 

1. THE METHOD OF ELECTRODEPOSITING A CHROMIUM PLATE DIRECTLY ON THE SURFACE OF AN ALUMINUM ARTICLE, WITH COMPRISES IMMERSING SAID ALUMINUM ARTICLE IN A PLATING BATH COMPRISING A MIXTURE OF 15-45 OZ./GAL. CRO3, H2SO4, THE RATIO OF CRO3 TO H2SO4 BEING IN THE RANGE OF 80/1 TO 200/1, WATER, AND 3.0333-6.0 OZ/GAL. SIO2 AS PARTICLES OF SUBMICROSCOPIC DIMENSIONS CARRIED IN SUSPENSION IN SAID BATH, AND ELECTROLYZING SAID BATH TO PLATE SAID ARTICLE. 